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1991 HEPP Conference: LEP Precision Is Main Feature 1991 HEPP Conference ments of heavy quarks and τ leptons [τb = (1.33 ± 0.05 ± 0.06).10 -12 s, ττ = (302.5 ± 5.9).10-12 s] from various experiments LEP Precision is Main Feature now agree. Furthermore, the new data have improved the knowledge of the Cabbibo- Kobayashi-Maskawa matrix elements. The The Divisional Conference of the High J. Carter summarized precision tests of way CP violation is achieved in nature still Energy and Particle Physics Division takes the standard model and the following set of remains an open question. Two experiments place every two years. As the IUPAP Lepton- measurements serves as an example of the at CERN and at Fermilab, employing neutral Photon Symposium was due to be held in high level of accuracy with which the funda­ kaon beams, do not yet allow a conclusive Europe, it was felt that one should accom­ mental parameters for electroweak inter­ answer to the size of the CP violating para­ modate the large overlap in physics topics actions have been determined : meter Re (ε'/ε), but new experiments with a with the EPS High Energy Physics Confe­ higher accuracy are planned (J.M Gerard). rence by organizing only one meeting in The four LEP experiments have also Geneva. The 1991 Conference was thus searched for new phenomena beyond the organized jointly by EPS, IUPAP and CERN, standard model and, In particular, have and took place in the Geneva Conference The electroweak Z parameters have been looked for the appearance of new particles. Centre with over 1000 participants from used to evaluate the number of light neu­ M. Davier demonstrated that as compared around the world. The excellent organiza­ trino generations, which is found to be to previous experiments, ail limits have been tion was in the hands of Luigi di Leila (CERN) 2.99 ± 0.05. Furthermore, lepton universa­ considerably improved owing to the large as Chairman of the international organizing lity has been established to a level well statistics and the increase in energy. Much committee, and Keith Potter (CERN), the below 1%. The precision of the experimen­ low energy territory is excluded as the head of the local organizers. tal data is also sufficient to detect effects of Higgs must be heavier than 57 GeV, as The HEPP Division awards the EPS High electroweak radiative corrections, which opposed to an unambiguous limit of 52 MeV Energy Physics Prize on the occasion of depend on the mass of the top quark. J. Ellis for only one pre-LEP experiment. The 57 its biennial conference. In Geneva, Nicola gave a limit for the top mass Mt of 125 ± GeV limit will probably be pushed out to Cabbibo (Rome University) was honoured 30 GeV, and a more conservative estimate 60-65 GeV in the next two years at LEP. for his fundamental contributions to the with input from other electroweak pro­ Physics beyond the standard model such as theory of weak interactions leading to quark cesses of 80 GeV < Mt < 200 GeV. He compositeness, SUSY and GUT's which we mixing (see appreciation on page 181). also described how a first attempt in 1990 may meet at higher energies were discus­ The conference itself was divided into to use high-precision LEP data to constrain sed by G.G. Ross. With regard to the 17 keV two parts. The first three days (25-27 July) hadronic uncertainties in placing bounds on neutrino problem, it can be accommodated comprised parallel sessions on 17 topics, the mass of the Higgs gave 1.8 Gev < MH consistent with most conservative cosmo­ where contributions from individual groups < 6 TeV with an optimal value of = 40 GeV. logical bounds, but accounting for decay of were presented, or the most recent develop­ The latest LEP data give rise to speculations this heavy neutrino requires new physics. ments In a particular field were summarized that the Higgs is "light" (< 200 GeV). At high energies, hard hadronic processes by rapporteurs. The second half (29 July to T. Hebbeker concentrated on the QCD are well described in the framework of per­ 1 August) was devoted to plenary talks, studies in hadronic decays of the Z boson. turbative QCD. However, the fact that essentially summarizing the discussions in The data are well described by the standard hadronic interactions are, in principle, very the parallel sessions. nonabelian QCD gauge theory with a strong complicated and difficult to interpret beco­ coupling constant as = 0.120 ± 0.007 at mes apparent from the study of soft hadro­ LEP Precision the Z° mass. The impact of the new data nic interactions (P. Landshoff) and hadron Physics at the Conference was domi­ on the status of QCD was summarized by spectroscopy (R. Landua). There is still nated by the excellent data presented G. Martinelli. Some observables suggest hope, nevertheless, that thermodynamic by CERN's four LEP experiments (ALEPH, that the domain of computability, made less methods may give some insight into long DELPHI, L3 and OPAL). Together they have easily accessible by the relatively large range quark-gluon interactions by studying collected more than one million Z° decays, value of as, can be studied experimentally. very high energy, heavy ion collisions. As which have been analyzed with respect to Heavy flavour physics at LEP was covered pointed out by H. Satz in his review talk, various physics aspects, including precision in a talk by P. Roudau, while M. Danilov the next generation of accelerators could tests of the standard model, QCD studies, summarized the investigation of heavy heavy flavour physics, and the search for quark decays from experiments at lower have the potential to create a quark-gluon new particles. centre of mass energies. Lifetime measure­ plasma. New Facilities S.L. Glashow speaking at the final plenary session of the Joint International Lepton-Photon Progress in high energy physics is based Symposium and Europhysics Conference on High Energy Physics. on the development of new accelerators and the steady improvement in detector techniques. The HERA ep collider at DESY in Hamburg is the next large facility which is at present being commissioned and will come into operation next year (B.H. Wiik). C. Rubbia thought other accelerator projects such as SSC, LHC and B factories were still several years ahead of us. Definitive studies of solar phenomena requiring the simulta­ neous observation in real time of neutrino scattering and ß decays are now being im­ plemented in the passive detectors ICARUS and Super-Kamiokande (the Japanese Go­ vernment approved in May an upgrade of Kamiokande from 3 to 50 k tonnes). As QCD is probably the correct description for nuclear interactions, he looked forward to its being thoroughly tested at HERA, to 180 Europhys. News 22 (1991) covered by P. Jenni, who quoted test beam measurements with a spatial resolution down to 20 µm : detectors with such a reso­ lution would be used for lifetime measure­ ments at colliders. Passive Experiments New results from non-accelerator experi­ ments gave rise to animated discussions. The observed flux of energetic solar neutri­ nos seems to be significantly lower than model calculations would predict. If this is verified, the observations could be the first hint of a new physics beyond the standard From the left, W. Schmidt-Parzefall, P. model. It therefore remains Important to Professor Y. Yamaguchi, IUPAP President- Schlein and B.H. Wiik in front of the EPS measure the neutrino flux from the sun with Designate, with Dr. B. Richter, Director of stand at the 1991 LP-HEP Conference. low threshold detectors (first generation SLAC, seated in the background. detectors using a chloride solution only some extent at LEP and in heavy ion colli­ measured 8Be and 7B fluxes in the solar a 51Cr source, the measured flux was low sions at CERN-SPS and later at RICH. He neutrino spectrum). Two experiments with (< 79 Solar Neutrino Units — SNU — at underlined the importance of supercompu­ Ga as the detector medium (GALLEX and 90% confidence limits) as compared to the ter calculations at APE in Rome and infor­ SAGE) are on the verge of obtaining data theoretically predicted rate of 132 SNU. mation on the spectroscopy of bound ha­ within the next few years. SAGE In the Finally, two new experiments are In prepara­ dronic states from LEAR and planed e + e- USSR involving 60 t of metal rather than a tion (BOREX and SNOW) for measuring the colliders. Finally, while the K meson system solution of the chloride is more compact, solar neutrino spectrum. is too complicated to distinguish between but extraction of the 71Ge produced is more The existence of a 17 KeV neutrino (page competing models for CP violation, mixing complicated. However, a background radia­ 182) gave rise to controversial discussions in the B quark system has opened up a new tion problem has held up calibration of which only further experiments will be able field to search for CP violation. Powerful GALLEX with a 51Cr source. It arose owing to resolve (talks by B. Barish and R. Möss- new sources of B quarks are thus planned to the generation during storage of 68Ge by bauer). In an audience dominated by experi­ at LEP, a future e + e- collider, SSC, and at cosmic radiation. Heating the solution was mental physicists, C. Itzykson succeeded in LHC. evaluated In "hot runs" that started in explaining to a largely non-expert commu­ New accelerator techniques for high Iate-1990 and the cool down last April re­ nity the most recent trends and develop­ energy facilities, notably linear and circular sulted in a satisfactory background level. ments In abstract field theory.
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